Human visual system as part of advanced neuroscience research

1. The visual system.
By Alex Antoniades
and Robert Joaquim.

2. Overview
 The visual system mechanism.
 Light.
 The eye.
 Structure and visual fields.
 The receptors role.
 The connection to Visual cortex.
 Visual cortex and other areas of occipital lobe.
 Experiment
 Prenatal visual system – Development
 What where pathways.
 How the visual system develops.
 Improvement of vision.
 Cortical development in newborns.
 Externality.
 Color vision.
 Sex differences.

3. Our visual world.
 Radiant electromagnetic
energy – Light.
 Light is similar to sound
waves (rise & fall).
 The wavelength of light
(frequency) denotes
different color.
 Intensity of frequency
refers to brightness.

4. The eye
 Each cell in visual cortex
receives its input via the
retina, from a particular
part of the visual field.
 This is the known as the
receptive field of the cell.
 ‘Retinotopy‘ is the term
referred to stimulated
cells in the retina and the
visual cortex that are
linked together.

5. Pictures of retina.

6. Structure and visual fields.
 Two visual fields in each
eye. Right and left.
 Lining of eyes
transforms light into
neuronal signals.
 Information from right
half of the visual field
goes to the left primary
visual cortex, and vice
versa.

7. Structure and visual fields, cont..
 Information from the
center of our visual fields
goes to the most
posterior part of the visual
cortex to be represented.
 Information from
peripheral parts are
mapped anteriorly in the
primary visual cortex.
 Less area is devoted in
visual cortex for
peripheral view receptors.

8. The receptors role.
 Rod cells : 1000 times more
sensitive to light (night vision), 20
times more than cone cells.
 Cone cells : 3 different types,
used for color vision, are not used
in dark.
 Rod cells have more membranous
disks than cone cells, they are
used for containing photopigment
as a part of the outer segment of
the cell.

9. The receptors role, cont..
 The fovea contains only
cones (colored vision,not
used in dark) and the
surrounding contains
mainly rods (used for
night vision).
 When light energy occurs
on the cells, they are
excited throughout a
chemical process known
as phototransduction.

10. The receptors role, cont..
 Absorption of light activates the
cell. The information passes to
the bipolar cell which synapses
with the photoreceptor that is
excited. The bipolar cell after
modifying the signal, it passes the
information to the ganglion cell.
Amacrine and horizontal cells are
changing the signal when still in
the bipolar cell.

11. The connection to Visual cortex.
 Signals from right retinas of both
eyes travel through the optic
nerve, optic tract, and optic
radiations to the primary visual
cortex of the right hemisphere,
whereas signals from the left
retinas travel to the left
hemisphere.
 When ganglion cell axons pass
along the optic nerve they
combine to form the optic
chiasm (centre of picture).

12. The connection to Visual cortex,
cont..
 Some axons from optic tract form
connections with hypothalamus
(learning).
 Most of the axons after the optic
chiasm pass to Lateral
Geniculate Nucleus of dorsal
thalamus and then to the primary
visual cortex also know as area
17, and area V1 of the occipital
lobe.

13. Lateral Geniculate Nucleus.
 Has 6 layers, starting with
ventral (bottom).
 Layers 1 & 2 contain bigger
neurons -Magnocellular.
 Layers 3 to 6 contain smaller
neurons – Parvocellular.
 80% of the input in LGN
comes only from the visual
cortex.
 It is projected to the primary
visual cortex.

14. Primary Visual Cortex.
 9 different layers.
 Cell arrangement highly
complex.
 3 separate processing
channels along Calcarine
sulcus (line of Gennari).
 M-channel – Motion.
 P-IB-channel – Shape.
 Blob-channel – Color.
 Striate cortex has around
1000 modules

15. Experiment.
Pick up one of the slips of paper and hold it with your right
hand. Close your left eye with your left hand.
Focus on the left side of the slip on the color face, and watch the
right hand side face disappearing as you move it closer to you
eye.
Blind spot.
Color blind spot.
Pick up one of the slips of paper and hold it with your right
hand. Close your left eye with your left hand.
Focus on the right side of the slip on the grey face, and watch
the left hand side changing color as you move it closer to you
eye.

16. What & Where pathways
 Important for parallel processing
 Where stream – deals with visual space,
detects speed, direction, and location in 3-D
space. Directs eye movements to follow
targets
 What stream – identifies objects or familiar
features, detects colour, shape, and fine detail

17. How the visual system develops
 Visual development begins 4th
week of embryonic life
 1st
= neurons & synapses in retina, 2nd
= subcortical
visual areas, 3rd
= primary visual cortex, 4th
= higher
visual centers
 Takes months for whole system to be up & running
 Years before pathways are stabilised
 First optic tissue = 22 days after fertilisation
 By 5 weeks eye cups have developed

18. How the visual system develops
cont…
 8 weeks = eyelids form & fuse shut
 Retina neurons divide into layers for specific functions
 1st
layer to form, ganglion cells between 6 & 20 weeks
of gestation. Foveal cells formed by 14 weeks of
gestation
 Some rods & bipolar cells in peripheral parts of retina
still developing many months after birth
 All of 100 million neurons in primary visual cortex
formed between 14 & 28 weeks of gestation

19. How the visual system develops
cont…
 Synapses in primary visual cortex form in 5th
month. Process continues for another year at
10 billion synapses per day
 Evidence suggests ‘where’ stream develops
earlier than ‘what’ stream. By 4 months of age
‘where’ stream reaches max synaptic density
while ‘what’ stream takes another 4 months
 8 months after birth, primary visual cortex
reaches max synaptic density

20. The improvement of vision
 Fetuses 24 weeks postconception have been
seen to react in response to strong light
pointed at mother’s abdomen
 At 2 months old, baby’s cerebral cortex takes
over most visual tasks from subcortical circuits
 By 1 year old, baby’s vision nearly as good as
adult’s

21. Baby Audience
That’s how the presentation would look like if you were 4 weeks old

22. Newborn Vision
 Vision similar to looking out of frosted glass
 Can only focus between 7 & 30 inches, can
see some colour. Better vision at edge of
visual field
 See motion better than other visual features
due to ‘where’ stream
 Innate preference for faces or facelike objects

23. Newborn Vision cont…
 Track slowly moving objects
 Unsteady eye movements – saccades
 Smooth by 2 months
 3 to 6 months – baby can anticipate movements & focus
ahead
 Vision 30 times worse than adults
 Acuity improves a lot due to changes in retina & cerebral
cortex

24. Improving vision
Newborn 4 Weeks 8 Weeks 3 Months 6 Months

25. Newborn Vision cont…
 Fovea slowest part of
retina to mature
 Foveal cones short & fat
 After birth, cones grow
significantly longer &
slimmer
 Therefore, more cones &
increased sensitivity to
light

26. Cortical development in newborns
 At birth, few action potentials in response to visual
stimuli
 No. of active neurons increase due to rapid growth of
cortical synapses & myelin
 Hyperacuity starts at 4 months

27. Externality
 Emphasis on peripheral
vision
 Newborns rely on this
much more than adults
 Lasts up to 2 months
until fovea is mature &
cerebral cortex takes
over fine vision

28. Colour vision
 Colour vision improves as cones mature
 At 8 weeks colours can be differentiated as long as
large & bright e.g. apple & orange
 Blue cones aren’t so good at this age
 By 3 months, blue cones are as good as red & green
ones
 By 4 months, areas in primary visual cortex used for
processing colour are fully developed

29. Colour vision cont…
 Habituation experiments
 Babies find it easier to
remember colours than
shapes
 Older babies prefer
bright reds or blues
 Maybe related to 4 types
of colour opponent cells
in LGN or cortex

30. Sex differences
 Between 4 & 6 months of age, hyperacuity is
significantly better in girls
 Between 7 & 10 years of age, males perform better on
visual spatial tasks

31. Questions.
 1) In which part of the brain is the line of Gennari
located?
 2) How long does it take for fovea to mature in
newborns.
 3) What is acuity?